Using lower limb exoskeletons to assess motor performance: A systematic review

Abstract: BACKGROUND AND AIM
Lower limb exoskeletons are mainly developed for individuals with a decreased level of motor performance to provide support or maintain walking ability. Particularly active exoskeletons use sensors to detect their internal state to control movement trajectories. Due to the proximity and the direct connection of the exoskeleton to the human body, data collected from these sensors can be used to calculate user-related motor performance data such as joints range of motion (RoM) or step length. This information may further be used to provide a real time feedback on current motor performance level, adapt training plans, or evaluate rehabilitation progress. The aims of this review were to provide an overview of research studies on motor performance parameters measured by lower limb exoskeletons, and to analyze how these parameters were measured.

METHODS
We followed the guidelines outlined in the PRISMA Statement, and conducted a systematic review (PROSPERO registration: CRD42021274215) by searching PubMed, Scopus, and Web of Science databases in July 2021. All steps including title, abstract and full text screening based on predefined inclusion and exclusion criteria, data extraction and quality assessment were done independently by two authors.
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RESULTS
Overall, 39 studies were included in this review. 16 studies used lower limb exoskeletons to measure joint angles or RoM, 14 studies measured joint torques or strength, and 10 studies measured gait parameters such as gait phase detection, step length or speed. Furthermore, 9 studies assessed spasticity, and 4 studies assessed proprioception. To this end, both stationary (n=5) and mobile exoskeletons (n=18) were used. Most exoskeletons supported only one joint (hip (n=7) and ankle (n=7); all mobile). 3 stationary exoskeletons only supported knee and hip, and 6 exoskeletons supported the full leg (stationary n=2; mobile n=4). 14 studies examined exoskeleton-based motor performance assessments in terms of validity (n=14) and/ or reliability (n=6). The stationary exoskeleton "Lokomat" was used in nearly one third of all included studies.
CONCLUSIONS Our review shows that various motor performance paramters can be measured using built-in sensors of exoskeletons. Compared to manual (e.g., clinical) test procedures, motor performance measurement through a lower limb exoskeleton may be more objective and specific for certain parameters such as proprioception or spasticity. Of note, assessing motor performance using exoskeletons is feasible, but validity and reliability studies are still scarce. More research is thus needed before exoskeletons can be used in clinical or research settings to assess human motor performance. Furthermore, adequate familiarization protocols, specific test control algorithms, and non-disturbing hardware designs, are critical.
ACKNOWLEDGEMENTS AND FUNDING
This research was conducted as part of the JuBot project which is funded by the Carl-Zeiss-Foundation.